Cellulosic biomass is a significant renewable resource for the generation of sugars. Fermentation of these sugars can yield numerous end-products such as fuels and chemicals that are currently derived from petroleum. While the fermentation of sugars to fuels, such as ethanol is relatively straightforward, the hydrolytic conversion of cellulosic biomass to fermentable sugars such as glucose is difficult because of the crystalline structure of cellulose and its close association with lignin (Ladisch, et al., Enzyme Microb. Technol. 5:82 (1983)). Pretreatment, by means, including but not limited to, mechanical and solvent means, increases the susceptibility of cellulose to hydrolysis, presumably by breaking the lignin seal and disrupting the crystalline cellulose structure. This step may be followed by the enzymatic conversion of cellulose to glucose, cellobiose, cello-oligosaccharides and the like using enzymes that specialize in breaking up the β-1-4 glycosidic bonds of cellulose. These enzymes are collectively referred to as “cellulases”.
Cellulases are divided into three sub-categories of enzymes: 1,4-β-D-glucan glucanohydrolase (“endoglucanase” or “EG”); 1,4-β-D-glucan cellobiohydrolase (“exoglucanase”, “cellobiohydrolase”, or “CBH”); and β-glucosidase (β-D-glucoside-glucohydrolase), (“cellobiase” or “BG”). These enzymes act in concert to catalyze the hydrolysis of cellulose containing substrates. Endoglucanases randomly attack the interior parts and mainly the amorphous regions of cellulose, mostly yielding glucose, cellobiose, and cellotriose. Exoglucanases incrementally shorten the glucan molecules by binding to the glucan ends and releasing mainly cellobiose units from the ends of the cellulose polymer. β-glucosidases split the cellobiose, a water-soluble β-1,4-linked dimer of glucose, into two units of glucose.
There are several types of microorganisms that produce cellulases. These include fungi, actinomycetes, and bacteria. Cellulases from strains of the filamentous fungi Trichoderma sp., Penicillium sp., and Chrysosporium sp. have been particularly productive in hydrolyzing cellulose and cellulases derived from these strains have been previously used to hydrolyze cellulose. However, the cost of producing these enzymes along with their hydrolytic inefficiency under certain industrial conditions has been a drawback.
In order to maximize the hydrolysis of cellulosic substrates and enable commercial routes to end-product production (e.g., biofuels), it would be highly desirable to develop new cellulases and particularly new endoglucanases useful in the saccharification of biomass (e.g., cellulose containing substrates).